One of the most important requirements of a next generation wireless communication system is to support a high data rate. For this, various techniques such as multiple input multiple output (MIMO), cooperative multiple point transmission (CoMP), relay, etc., have been under research, but the most fundamental and reliable solution is to increase a bandwidth.
However, a frequency resource is in a saturation state at present, and various schemes are partially used in a wide frequency band. For this reason, in order to ensure a broadband bandwidth to satisfy a required higher data rate, a system is designed such that a basic requirement which allows separate bands to operate respective independent systems is satisfied, and a carrier aggregation (CA) is introduced. In concept, the CA aggregates a plurality of bands into one system. In this case, a band that can be independently managed is defined as a component carrier (CC).
To support growing transmission capacity, it is considered in the latest communication standard (e.g., 3GPP LTE-A or 802.16m) to expand its bandwidth to 20 MHz or higher. In this case, a wideband is supported by aggregating one or more CCs. For example, if one CC corresponds to a bandwidth of 5 MHz, four carriers are aggregated to support a bandwidth of up to 20 MHz. As such, a system supporting carrier aggregation is called a carrier aggregation system.
In the conventional carrier aggregation system, all carriers use the same type of frames. That is, all carriers use a frequency division duplex (FDD) frame or a time division duplex (TDD) frame. However, in a future carrier aggregation system, it is considered to use a different type of frame for each carrier. In addition, it is also considered to use a different uplink (UL)-downlink (DL) configuration while a TDD frame is used in a group of some carriers among carriers constituting the carrier aggregation system.
Meanwhile, a wireless communication system can use hybrid automatic repeat request (HARQ) in which retransmission data is transmitted when an error occurs in transmitted data upon receiving reception confirmation information on transmitted data, that is, acknowledgement/not-acknowledgement (ACK/NACK). In an HARQ process, a receiving side can perform decoding by combining data originally transmitted in a transmitting side with retransmitted data.
There is a need for a method of performing HARQ when the carrier aggregation system uses a different types of frame for each carrier or when some carrier groups use a TDD frame while using a different UL-DL configuration. More specifically, there is a need for a method of transmitting ACK/NACK by a user equipment.